A typical liquid crystal display (LCD) is capable of displaying a clear and sharp image through thousands or even millions of pixels that make up the complete image. The liquid crystal display has thus been applied to various electronic equipment in which messages or pictures need to be displayed, such as mobile phones and notebook computers. A liquid crystal panel is a major component of the LCD, and generally includes a thin film transistor (TFT) array substrate, a color filter substrate, and a liquid crystal layer sandwiched between the two substrates.
Referring to FIG. 6, part of a typical TFT array substrate 100 is shown. The TFT array substrate 100 includes a glass substrate 101, a gate electrode 102 formed on the substrate 101, a gate insulating layer 103 formed on the substrate 101 including the gate electrode 102, a semiconductor layer 104 formed on the gate insulating layer 103, a source electrode 105 and a drain electrode 106 formed on the gate insulating layer 103 and the semiconductor layer 104, a passivation layer 107 formed on the gate insulating layer 103, the source electrode 105 and the drain electrode 106, and a pixel electrode 108 formed on the passivation layer 107.
To decrease a resistance-capacitance (RC) delay time, the gate electrode 102 is generally made from copper (Cu) because copper has a low electrical resistivity. However, the TFT array substrate 100 having the copper gate electrode 102 frequently has two defects. First, an adhesion between the copper gate electrode 102 and the substrate 101 is not very strong. Therefore the gate electrode 102 is liable to peel off from the substrate 101. Second, the copper is easily ionized to copper ions by heat dissipating from the TFT. The copper ions may penetrate into the gate insulating layer 103, or even penetrate into the semiconductor layer 104. This copper ion migration contaminates the gate insulating layer 103 and the semiconductor layer 104, and deteriorates the performance of the TFT. For example, leakage current may occur or be increased.
Referring to FIG. 7 and FIG. 8, in order to overcome the defects described above, improved sandwich structure gate electrodes 112 and 113 have been developed. The sandwich gate electrode 112 includes an adhesive layer 118 formed on a substrate, a barrier layer 114 formed above the adhesive layer 118, and a gate electrode layer 116 sandwiched between the adhesive layer 118 and the barrier layer 114. The adhesive layer 118 can enhance the adhesion between the gate electrode layer 116 and the substrate. The barrier layer 114 can prevent copper ions from penetrating into an adjacent semiconductor layer. The three layers 118, 114, 116 are made from different materials. The sandwich gate electrode 113 has a similar sandwich structure as the gate electrode 112. However, the gate electrode 113 is made from materials different from those of the gate electrode 112.
In an etching process during manufacturing of the gate electrode layer 116, an etching speed varies according to the different materials of the three layers 118, 114, 116. This may lead to a trapezium structure of the gate electrode 112. When the etching speed of the gate electrode 112 decreases from the top barrier layer 114 to the bottom adhesive layer 118, a width of the gate electrode 112 increases from top to bottom, as shown in FIG. 7. If the etching is not complete, a short circuit is liable to be created or occur during operation of the TFT. On the other hand, when the etching speed of the gate electrode 113 increases from the top barrier layer 114 to the bottom adhesive layer 118, a width of the gate electrode 113 decreases from top to bottom, as shown in FIG. 8. During subsequent depositing processes, open circuits or poor quality covering are liable to occur.
What is needed, therefore, is a thin film transistor array substrate and manufacturing method thereof that can overcome the above-described deficiencies.